The field of the invention is that of bonding two substrates to one another by molecular adhesion. The invention generally concerns a process and bonding equipment. It extends likewise to the formation of a structure comprising a thin layer made of a semiconductor material upon a support substrate. To form such a structure, the typical procedure is to place a donor substrate in close contact with the support substrate, so as to effect bonding by molecular adhesion of the substrates to one another. This is then followed by the transfer of a part of the donor substrate to the support substrate to form the thin layer on the support substrate.
Bonding by molecular adhesion (either ‘direct wafer bonding’ or ‘fusion bonding’) is a technique that enables two substrates having perfectly flat surfaces (e.g., polished mirror surfaces) to adhere to one another, without the application of adhesive (gum type, glue, etc.). The surfaces in question are generally those of substrates made of an insulating material (for example, quartz or glass) or a semiconductor material (for example Si, GaAs, SiC, Ge, etc). Bonding is typically initiated by local application of light pressure to the two substrates when they are placed in close contact. A bonding front then spreads over the entire extent of the substrates in a few seconds.
The bonding energy obtained at ambient temperature is generally low enough relative to that observed between two solids in covalent, ionic or metallic connection. For numerous applications, however, bonding is reinforced by carrying out thermal annealing. In the case of a silicon surface molecularly bonded to another silicon or silicon oxide surface, the bonding energy reaches a maximum after a bonding reinforcing annealing carried out at temperatures on the order of 1100° C. to 1200° C.
In addition, to obtain satisfactory bonding of two substrates, the typical procedure prior to bonding is the preparation of one or both of the surfaces to be bonded together. Enhanced bonding is intended to increase the mechanical performance of the bonded substrates or to boost the quality of the bonding interface.
An example of such a treatment for increasing the mechanical behavior between the substrates during bonding is the preparation of the surfaces to be bonded with the aim of making them more hydrophilic. Within the scope of hydrophilic bonding, the following properties are preferred for the surfaces to be bonded.
the absence of particles;
the absence of hydrocarbons;
the absence of metallic contaminants
a low surface roughness, typically less than 5 Å RMS;
strong hydrophily, that is, a substantial density of Si—OH silanol bonds terminating the surfaces to be bonded together.
The preparation of the surfaces to be bonded is generally completed by utilizing one or more chemical treatments. By way of example of chemical treatment prior to (hydrophilic) adhesion, the following can be mentioned:                cleaning of RCA type, namely the combination of a SC1 (NH4OH, H2O2, H2O) bath adapted to shrinkage of the particles and the hydrocarbons and a SC2 (HC1, H2O2H2O) bath adapted to shrinkage of the metallic contaminants;        cleaning with an ozone solution (O3) adapted to shrinkage of the organic contaminants;        cleaning with a solution containing a mixture of sulfuric acid and oxygenated water (or SPM solution, Sulfuric Peroxide Mixture).        
The preparation of the surfaces to be bonded can likewise comprise mechanical preparation of the surfaces (light polishing, brushing), either alone or in combination with the chemical treatment.
As a complement to conventional methods of bonding by molecular adhesion, strong bonding techniques at low temperature have been developed more recently to make heterostructures (two materials of different types) more readily adhere to substrates comprising partially or totally manufactured electronic components (aka patterned substrate and structured wafer), or even for adhering substrates that are capable of being altered during annealing at high temperatures. Bonding by molecular adhesion with plasma activation is an example of such a strong bonding technique that can be carried out at low temperature. Exposure of one or both surfaces to a plasma prior to bonding allows strong bonding energy to be reached after relatively short reinforcing annealing of the structure (around 2 hours) at a relatively low temperature (typically less than 600° C.). As a reference for this teaching, the following articles can be mentioned:
“Effects of plasma activation on hydrophilic bonding of Si and SiO2” T. Suni et al., J. Electroch. Soc, Vol. 149. No. 6, p. 348 (2002);
“Time-dependent surface properties and wafer bonding of O2-plasma treated silicon (100) surfaces”, M. Wiegand et al., J. Electroch. Soc. Vol. 147, No. 7, p. 2734 (2000).
It is evident that the different techniques for surface preparation mentioned earlier systematically incorporate at least one humid stage, that is, at least rinsing of the surfaces by deionized water. The substrates are then dried, for example by centrifuging (dry spin). As a function of their degree of hydrophily, the surfaces of the substrates have, after drying, several monolayers of adsorbed water, with these monolayers being at the origin of the intermolecular forces responsible for adhesion during contacting of the surfaces.
Also, bonding by molecular adhesion of substrates to one another generally causes defects. Notable examples of such defects include defects of the bubble type (or bubbles) at the level of the bonding interface between the two substrates, as well as defects of edge type (or edge voids) at the level of the thin layer of the final structure (the thin layer on support substrate that is obtained after transfer). Bubbles are understood to be defects that result from gas and/or water combining at the bonding interface between the two substrates. Bubbles can appear after the application of a low budget thermal to the adhered structure (for example after application of thermal annealing at 200° C. over 2 hours) and are observable by inspection of the bonding interface using an infrared camera, or by acoustic microscopy. The bubbles will be responsible for the presence of non-transferred zones at the level of the final structure obtained after transfer. The article “Low-Temperature Wafer Bonding, Optimal O2 Plasma Surface Pretreatment Time”, by X. Zhang and J-P. Raskin in Electrochemical and Solid-State Letters, 7 (8) G 172-G174 (2004), illustrates the phenomenon of the formation of bubbles at the bonding interface. Edge voids are understood to mean defects which result from bonding and which are typically observed at the periphery of the final structure (generally in the form of a circular plate).
An application of direct bonding is that carried out within the scope of producing structures of the Semiconductor On Insulator or SeOI type, and in particular for Silicon On Insulator or SOI structures. Included within this scope of this invention are substrates to be bonded where at least one has a surface layer of oxide; by way of example, Si/SiO2 bonding or SiO2/SiO2 bonding which are typically undertaken to form a SOI structure.
There are three main methods for producing SeOI structures by direct bonding: to well known SMART-CUT® process, or the so-called BSOI (and BESOI) or ELTRAN® processes. A description of the processes associated with each of these methods can be found in the text entitled ‘Silicon wafer bonding technology for VLSI and MEMS applications’, by S. S. Lyer and A. J. Auberton-Hervé, IEEE (2002). Despite the accepted use of these processes, defects of the edge voids type, cause by the bonding step, can often appear after transfer of the thin layer from the donor substrate to the support substrate.
As is schematically illustrated in FIG. 1 in terms of forming a SOI structure, an edge void P is a hole (of a diameter typically between 100 μm and 1 mm) in the thin transferred layer which corresponds to a zone of the donor substrate not transferred to the support substrate A. The edge voids appear most often at the edge (peripheral zone) of the “thin layer on support substrate” structure (circular wafer); and they are usually located at a distance of typically between 1 mm and 5 mm of the wafer edge.
The edge voids are macroscopic defects connected to poor bonding at the edge of the wafers. They can be killer defects because, in the absence of a thin layer acting as an active layer for the formation of electronic components at the location of an edge void, no component can be manufactured at this location. Given the size of the edge voids, an electronic component comprising at least one edge void is necessarily defective.
In addition, a transfer process of the SMART-CUT® type is notably interesting in that it allows for recycling of the donor substrate. So when adhesion of a recycled donor substrate is completed (that is, a donor substrate already having served for removal and transfer of a thin layer; known as ‘refresh’ wafer), a greater number of edge voids is observed than when bonding of an original donor substrate is completed (i.e., one having never served to remove and/or transfer of a thin layer; known as ‘fresh’ wafer). This increased presence of edge voids tends to prohibit the recycling of such wafers, thus defeating one of the main reasons for using the SMART-CUT® process.
Since the presence of edge voids induces losses in terms of quality and yield, there is thus a need to prevent the formation of such defects. It has been proposed in European patent application EP 1 566 830 to limit the number of defects of void type at the edge of a SOI wafer obtained as a result of molecular bonding. According to this application, these defects are always located at a specific position relative to the center of the wafer, and seem to be due to the configuration of the edges of wafers. Therefore, to decrease the number of defects, this application proposes modifying the configuration of wafer edges during manufacture. More precisely, this application proposes modifying the curve of the edge drops, in regions ranging from 3 mm to 10 mm from the periphery of the wafer. This solution thus has the disadvantage of requiring previous mechanical intervention on the wafers.
Another application of direct bonding is that of Si/Si bonding of DSB type (Direct Si Bonding). As mentioned earlier, defects of bubble type are all the same capable of appearing at the bonding interface. One solution for reducing the formation of bubbles in this process consists of producing plasma activation of the surfaces to be bonded, so as to obtain good adhesion energy, but this solution has not been found to be entirely satisfactory for reducing the number of bubbles at the bonding interface.
Thus, improvements in these type processes are desired and necessary.